Conventionally, in a heat-fixing device (fixing device) provided in a image forming apparatus employing a electrophotographic system or an electrostatic recording system, a so-called heating roller type fixing device is widely used. In the fixing device, a recording material carrying the unfixed toner image is passed through a nip provided between a fixing roller and a pressing roller which are press-contacted to each other and are rotated by which the toner image is fixed on the recording material as a permanent image.
On the other hand, a film heating type fixing device has been put into practice, in which no electric power is supplied to the fixing device during a stand-by period, by which the electric energy consumption is minimized. Such a film heating type fixing device proposed and put to practical use as disclosed in Japanese Laid-open Patent Application Sho 63-313182, Japanese Laid-open Patent Application Hei 2-157878, Japanese Laid-open Patent Application Hei 4-44075 and Japanese Laid-open Patent Application Hei 4-204980, for example.
FIG. 2 shows a typical film heating type fixing device. A fixing nip N is formed by a heater 204, a pressing roller 202 supported by a heat-insulative holder 205 and a resin or metal fixing film 203 (fixing film) having a high heat conduction and sandwiched therebetween. The unfixed toner image formed and carried on the recording material is introduced into the fixing nip N and is heated and fixed. In order to provide a sufficient width N of the fixing nip to form a satisfactory fixed image, the fixing members including a heater 204 and a fixing film 203 are urged to the pressing roller 202 by an urging spring 206 or the like against the elastic of the pressing roller 202 In order to stably provide the fixing nip width N which is substantially uniform along the longitudinal direction of the fixing member, a pressure substantially uniform along the longitudinal direction of the heat-insulative holder 205 is applied through a metal stay 207 having a reverse U shape In addition, a structure in which a core metal at a end of the pressing roller is provided with a electroconductive rubber ring 209 such that the film potential is stabilized, is put into practice.
Recently, however, there are demands in an image forming apparatus such as a copying machine or a printer, toward a high printing speed, quick start, power save or downsizing. Because of the speed up of parts, the fixing temperature rises, and in order to accomplish the quick start, the improvement in the thermal responsivity of the heater and the reduction of the low thermal capacity thereof are intended. As a result, the temperature difference becomes large between the area in the fixing nip where the recording material exists (sheet passing area) and where the heat of the fixing device is deprived by the recording material and the area where the recording material does not exist (non-sheet-passage-part) and where the heat is not deprived. Therefore, when a recording material (small size sheet) having a relatively small width as compared with the length of the fixing device is fed into the fixing device, the temperature difference in the fixing device along the longitudinal direction is large.
This means that a temperature difference between the proper fixing temperature for the recording material and the destruction temperature of the fixing device, that is, the margin is small. At present, in order to reduce the temperature difference, as compared with the case in which a relatively large recording material (full size sheet) is processed, when a small size sheet is processed, the printing speed is lowered (throughput down) to provide a time period for reducing the temperature unevenness, in many examples. In the actual situations, limited numbers of sheets are processed randomly, but in conventional devices, the setting of the throughput down is determined supposing that a large amount of the small size sheets are continuously outputted. The result is that for the actual use of the device, the margin against destruction is relatively large. Thus, in the case of outputting small size sheets, the throughput down as compared with the case of the large size sheets is significantly large, which is not desirable for the users.
Prior art solving this problem proposes that a plurality of heat generating elements having different lengths are prepared, and the different heat generating elements are used correspondingly to the different lengths of the recording material. An example of such a structure is disclosed in Japanese Laid-open Patent Application 2006-84805. However, with this structure, the problems of complicated structure of the device and the resulting cost increase arise, and therefore, it is difficult to employ it in a low cost machine.